Measurement instruments are commonly employed for determining various characteristics of radio frequency (RF) signals. One example of such a measurement instrument is a spectrum analyzer.
FIG. 1 shows a functional block diagram illustrating one embodiment of a front end section of a spectrum analyzer 100. Spectrum analyzer 100 includes an adjustable input attenuator 102, an attenuation control 104, a switch 106, switch control 108, low band circuits 110, a tunable filter 112, a tuning control 114, and high band circuits 116.
Low band circuits 110 and high band circuits 116 process signals at different ranges of frequencies. In one embodiment, for example, low band circuits 110 process “low frequency” signals having frequencies less than about 3.6 GHz, and high band circuits 116 process “high frequency” signals having frequencies from about 3.6 GHz to 26.5 GHz.
In one embodiment, for example, attenuation control 104 may operate in response to a user input that indicates a level of input attenuation to be applied corresponding to a signal level being applied to the input of spectrum analyzer 100. Switch control 108 may operate in response to a frequency selection made by a user to indicate a band of interest where an input signal is to be measured. In one embodiment, the functional blocks of attenuation control 104, switch control 108, and tuning control 114 are implemented in whole or in part by a processor, or under control of a processor as a result of user input(s) to control operations of spectrum analyzer 100.
Typically, tunable filter 112 is an yttrium-iron-garnet (YIG) tuned filter (YTF). Accordingly, to provide a concrete example and explanation of an operation of spectrum analyzer 100, hereafter tunable filter 112 will be referred to as YTF 112. YTF 112 has a tunable passband where signal loss is relatively low, while signals at frequencies outside the passband have a substantial loss. Beneficially, the passband of YTF 112 is tunable across the frequency band of high band circuits 116 (e.g., from 3.6-26.5 GHz) in response to a tuning current supplied by tuning control 114. The passband of YTF 112 may be defined in a variety of ways, but is often defined as the being bounded by the upper and lower frequencies beyond which the attenuation of YTF 112 is more than X dB (e.g., 4.5 dB) greater than the attenuation at the center frequency of the passband. The frequency range spanned by the passband of YTF 112 is referred to as its bandwidth, and in practice the bandwidth may vary as a function of the frequency to which YTF 112 is tuned. As an illustrative example, in one embodiment of YTF 112, the bandwidth is in a range of 50 MHz when YTF 112 is tuned to 3.6 GHz, and in a range of 100 MHz when YTF 112 is tuned to 18 GHz.
FIG. 2 shows an example of a passband frequency response of one embodiment of YTF 112. In FIG. 2, reference numerals 201 and 202 denote the lower and upper −4.5 dB frequency points that are used, in this example, to define the passband. Reference numeral 203 indicates the peak frequency, and reference numeral 204 indicates the center frequency of the passband. As can be from FIG. 2, in practice the passband of YTF 112 is not perfectly flat. Instead there is typically an amplitude variation or ripple across the passband.
As noted above, the passband of YTF 112 is tuned by application of a tuning current, for example supplied by tuning control 114 in FIG. 1. In general, the frequency-versus-tuning-current response of YTF 112 is, to a first order or approximation, linear. However, for a high quality measurement instrument, the deviation of the tuning response of YTF 112 from a perfectly linear response is significant. In particular, it can be seen from FIG. 2 that a relatively small deviation in the tuned frequency of YTF 112 may produce a significant change in the amplitude response. When spectrum analyzer 100 is used to measure a signal that passes through YTF 112 and high band circuits 116, this amplitude variation of YTF 112 must be accounted for and corrected.
Accordingly, in some embodiments, spectrum analyzer 100 is subjected to a calibration cycle. An externally-generated signal having a known calibration frequency (i.e., traceable back to known and accepted standards) is supplied to the input of spectrum analyzer 100, and a tuning current is applied to YTF 112 that should tune the center frequency of YTF 112 to the calibration frequency if the frequency-versus-tuning-current response of the YTF was perfectly linear. The actual tuning response of YTF 112 is measured. The process is repeated at K different calibration frequencies (e.g., K=200) spanning the operating frequency band of YTF 112 and high band circuits 116. The K measured data points measured during such a calibration then may be used during normal operations of spectrum analyzer 100 to correct for the non-linearity in the tuning response of the YTF.
Unfortunately, the tuning response of a YTF is known to vary with time and according to environmental factors including temperature—including in particular the temperature of the coupling loops in the YIG device.
FIG. 3 illustrates some measured data that plots changes in the frequency-versus-tuning-current response for one embodiment of a YTF as a function of temperature, and further as a function of operating frequency. As can be understood from FIG. 3, at any given tuning current level, the actual tuned center frequency of the YTF varies with temperature. Furthermore, the amount of variation in the center frequency increases with increasing operating frequency. So, for example, it can be seen that for a tuning current that produces a nominal center frequency of 3.6 GHz, the actual center frequency varies over a range of about 3 MHz over a temperature range of 20-55° C., while at a nominal center frequency of 25 GHz, the actual center frequency varies over about 27 MHz over a temperature range of 20-55° C.
In fact, YTFs drift with age and drive history in ways that have poor predictability.
In current practice, the YTF is centered before a calibration point is measured, and it is centered before verification, prior to making a measurement, but those centerings are not identical, leading to calibration errors. Furthermore, there is a tradeoff between having lots of calibration frequencies, for better prediction of the response versus frequency, and having few calibration frequencies, for lower calibration costs. This tradeoff is inconvenient.
What is needed, therefore, is an improved method for compensating for tuning inaccuracies in a tunable filter in a measurement instrument, such as a spectrum analyzer. What is also needed is a measurement instrument such as a spectrum analyzer that incorporates such an improved method.